I was recently at a customer site asked to look at an issue with users unable to open Shared mailboxes in Outlook following an upgrade of their Outlook 2010 to Outlook 2016.

The customer had just migrated to Office 365. The user with permissions could originally access these mailboxes when the mailboxes where On-Premises and could still access when the mailboxes were migrated to Exchange Online as user mailboxes and given a license. I wasn’t involved in the originally migration so wasn’t sure why the mailboxes were not converted to Shared mailboxes On-Premises before the move.

The customer was consolidating their licenses and as these mailboxes where not user mailboxes decided to convert the mailboxes to Shared mailboxes as these do not require a license. The client had also deployed Office ProPlus to replace their Office 2010 suite.

On investigations it wasn’t an issue with ALL shared mailboxes. There didn’t appear to be a pattern to which could be accessed, and which couldn’t. I checked the permissions and all permissions were set correctly. Automapping function was working and the user could see the mailbox appear in their outlook. After further investigations I found the following:

The Cause:

The mailboxes with issues were originally given an “Office 365 F1” license. This license does not allow connectivity from an Outlook client over MAPI. I confirmed this by looking at the mailbox properties and MAPI was disabled. The Outlook 2010 users had been connecting on RPC over HTTP.

The Fix:

  1. Apply a temporary Exchange Online Plan 1 license to the shared mailboxes
  2. Run the cmdlet set-casmailbox -MapiEnabled $true
  3. With Mapi Enabled I could then remove the Exchange Online Plan 1 license

Users could then access the shared mailbox!